FUNDAMENTAL PROBLEMS. 75 



At the critical frequency the reactance (a>L i/coC) is equal 

 to zero, equation (8) of Art. 27 reduces to /= EfR, and equation 

 (9) of Art. 27 shows that 6 is zero. That is, at the critical fre- 

 quency the reaction of the condenser annuls the reaction of the 

 inductance, and, after the current becomes established, the circuit 

 behaves as if it contained resistance only. The annulling of in- 

 ductive reaction by capacity reaction means simply that the elec- 

 tromotive force, coLI, Fig. 59, which overcomes inductance is 

 equal and opposite to the electromotive force, f/coC, which over- 

 comes the reaction of the condenser, so that the electromotive 

 force which acts on the circuit has to overcome resistance only. 



The very sharply defined frequency at which resonance occurs 

 is shown in Fig. 66. The ordinates of the curve represent the 



effective values of the current produced by a harmonic electro- 

 motive force of 200 volts (effective) in a circuit like Fig. 57 in 

 which R= 2 ohms, = 0.352 henry, and C= 20 X io~ 6 

 farad ; and the abscissas represent various frequencies. At very 

 low frequencies the value of 1/coC is very great, and the cur- 

 rent is limited by the condenser reaction ; at the critical fre- 

 quency of 60 cycles per second, the current is equal to E\R ( = 

 100 amperes effective) ; and at very high frequencies the value 



